1. Field of the Invention
The present invention relates generally to the field of composite electrical insulators, and more particularly to methods of assembling and manufacturing a composite electrical insulator comprising an insulator sub-assembly and a metal end fitting, and the resultant composite electrical insulator.
2. Description of the Related Art
For quite some time composite electrical insulators have been used to insulate high tension wires from the towers to which they are anchored. Over time this field has become fairly complex as engineers have continually improved these insulators. In recent years, it also been a priority to improve the ease with which these insulators are produced. For example, U.S. Pat. No. 5,563,379 to Kunieda et al., incorporated by reference herein, shows, with reference to FIG. 1 herein, a composite electrical insulator 100 capable of maintaining good water-tightness between a metal fitting 102 and a sheath 104 without an increased clamping force. The metal end fitting 102 has a sleeve portion 106 defining a bore 107 in which the end portion of an FRP rod 108 is received. The FRP rod 108 is covered by the sheath 104, which has two circumferential ridges 110 on its outer surface. As shown in FIG. 2A, the circumferential ridges each have an outer diameter (d.sub.2). The inner diameter (d1) of the bore 107 defined by the sleeve portion 106 is greater than the outer diameter (d2) of the circumferential ridges 110. In order to prevent water from leaking into the space between the sleeve 106 and the ridge 110, as shown in FIG. 2b, Kunieda et al. crimped the sleeve portion 106 onto the circumferential ridges 110 to force intimate contact between the circumferential ridges 110 and the inner surface of the bore 107 of the metal fitting 102. Once assembled, the circumferential ridges 110 served as O-rings which prevented the water from penetrating inside the metal fitting 102. That is, when the sleeve portion 106 of the metal fitting is applied with a moderate crimping force, the circumferential ridges 110 are compressed by the metal fitting 102 into conformity with any unevenness on the inner surface of the metal fitting 102, thereby maintaining the desired water-tightness for a long period.
However, one problem with manufacturing an insulator according to this method is that if there is any variance in the dimensioning of the bore 107 and the circumferential ridges 110, the ridges 110 may not completely contact the inner surface of metal fitting 102. Similarly, any eccentricity between the sleeve portion 106 and the bore 107 may result in a gap between the sleeve 106 and ridges 110. In either case, there is a chance water may leak into the gap between the sleeve 106 and the ridges 110. This is dangerous since water may possibly penetrate the boundary between the FRP rod 108 and the sheath 104, and the electrical insulating performance of the insulator will deteriorate so much that electrical discharge (i.e., flashover) will occur. As a result, the very function these insulators are intended to perform (i.e., insulation) is destroyed. Such water leakage can also cause rusting of the inner surface of metal fitting 102, which in turn relaxes the crimping force between the rod/sheath insulator subassembly and metal fitting 102.
The only way to ensure a good fit between the sheath and the metal fitting and thus guard against such water leakage is to ensure extremely precise dimensional control of the circumferential ridges 110 and the inner surface of the metal fitting 102. The former requires precisely machined molds, and the latter requires precise machining of the metal end fitting. Both complicate the manufacturing process and increase cost.
Additionally, because the outer diameter (d2) of the circumferential ridges 110 is less than the inner diameter of the bore defined by the metal fitting 102, that portion of the metal fitting 102 overlapping the circumferential ridge 110 must be crimped to compress the ridge 110 and form a good seal. This crimping step is in addition to the crimping step used to plastically deform the metal fitting 102 around the FRP rod 108. It would be desirable to eliminate this second crimping step to make the insulator easier and cheaper to assemble.
Thus, there is a clear need in the industry for a composite electrical insulator which is more easily and securely assembled to a metal end fitting member. By eliminating the associated need for high precision dimensional control and two crimping steps, manufacturing time and expense could be significantly reduced.